Circuit and method for trimming integrated circuits

ABSTRACT

A programmable after-package, on-chip reference voltage trim circuit for an integrated circuit having a plurality of programmable trim cells generating a programmed sequence. A converter is provided to convert the bit sequence into a trim current. The trim current is added to an initial value of a reference voltage to be trimmed, as generated by the integrated circuit. Once the correct value of the trim current is determined, isolation circuitry is programmed to isolate the trim circuitry from the remainder of the IC, thereby freeing the logic and package pins associated with the IC for use by users of the IC. The preferred trim circuitry includes fuses which are blown in accordance with a bit value supplied to the trim cell to permanently fix a trim current value, once a best fit value is determined.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a circuit and method for trimmingintegrated circuits, and more particularly, the present inventionrelates to a circuit and method for trimming packaged integratedcircuits without requiring additional package pins or logic toaccomplish same. Particular utility for the present invention is intrimming a reference voltage associated with a mixed-signal IC; althoughthe present invention is equally applicable for any IC where a precisereference voltage is desired: for example, accurate voltage controlledoscillator, high precision DAC, accurate current generator, etc.

2. Description of Related Art

In manufacturing analog (mixed signal) integrated circuits, the basicbuilding blocks are usually not accurately controlled by themanufacturing process as may be desired. For example, capacitors andresistors may have the wrong value, and MOS transistors may have thewrong gain setting. There are too many variables in the manufacturingprocess to yield absolute predictable results. Yet historically analogcircuits often require very accurate voltage references, frequencyreferences, and accurately ratioed elements.

To compensate for the process variability, many electronic circuits useanalog trimming during test to set resistor values as necessary forproper operation of the circuit. A typical trimming technique utilizes aresistor ladder comprising a series of serially coupled resistors eachin parallel with either a fuse or anti-fuse. A fuse is a device that issubstantially an electrical short until it is blown open. An anti-fuseis an electrical open until blown when it becomes substantially anelectrical short.

The fuse-blowing approach may take several forms, each with its ownshortcomings. Laser fuses may be used directly across each resistorelement in the ladder to enable and disable conduction through theresistor. During test, certain resistors are selected to open the shuntelement thereby adding resistance to the serial path. The resistorladder should be adjustable at wafer test over a range from say 10 to2,560 ohms in 10 ohm increments.

The analog trimming may be performed iteratively, i.e. test, trim, test,trim, to measure the effect of the course trim and determine thenecessary fine trimming. For iterative trimming, a laser trim system istypically installed on the wafer tester to alternately test and trim.However, one laser system per tester is very expensive. The laser isoften in an idle state waiting for the tester. Moreover, if either thetest system or laser breaks down both are inoperative.

An alternate approach is to use a zener anti-fuse across the resistorladder. Such an element can be cheaply trimmed on the tester so thatiterative testing can be done in one pass on the tester. Zeneranti-fuses require large voltage to program. Such a voltage placed onthe chip can affect the rest of IC especially for low voltage operation.Therefore, each anti-fuse requires its own external pad and probe cardneedle. This restricts the programming bit count to say 5-10 bits beforethe die area for test pads and complexity of the probe card requirementsbecome prohibitive.

In general, iterative testing is a slow and expensive process.Consequently, many trimming techniques utilize only a single pass toevaluate which resistors in the serial string should be included toachieve the desired analog circuit operation. Thus, as result of a testmeasurement, the user blows the shunt fuse elements whereby the circuitis expected to operate as planned. The process of blowing the fusestypically involves laser trimming off-line from the test set to cut thepoly material and open the shunt element. The circuit may be returned tothe test set to verify proper trimming. If the subsequent testing shouldfail, the part is typically discarded since it is difficult to patch theshunt fuse elements.

Moreover, these processes are performed at a wafer level, i.e., beforepackaging of the IC and require probe cards, long cables, etc., which isa labor and time intensive task for each IC. During the packagingprocess (e.g., die, cut and ceramic or plastic encapsulation), the IC issubject to mechanical and chemical stress which can again alter thecomponents that have been trimmed by the wafer trimming processes,rendering wafer-trimming an unattractive alternative. One partialresolution to wafer-level trimming procedures can be found in U.S. Pat.No. 5,079,516, issued to Russell et al. This patent discloses an afterpackage (i.e., post-assembly) trimming circuit and method for a LF155BIFET® monolithic JFET input operational amplifier, that operates tocorrect any discrepancies of a wafer trimming process performed beforepackaging of the IC. The type of JFET IC described in this patentincludes balance package pins (38 and 39, FIG. 2) which are normallyused to attach an external potentiometer to adjust the offset voltageafter packaging. The '516 proposes, however, the addition of on-chiptrim circuitry which effects an internal trim procedure, utilizing thebalance pins already provided. The '516 also isolates the balance pinsfrom the rest of the circuit after trimming, so that the trim valuecannot be altered by an accidental input on the balance pins by theuser. However, once trimmed the balance pins remain inactive, andcannot, by design be utilized by the IC. Thus, the balance pins remainas wasted real estate space on the IC, an important consideration whenminimizing package pins and chip “real estate”. Additionally, newer JFETIC of the type described in the '516 patent do not have external balancepins, and thus, cannot use the trim process proposed in the '516, andmust instead rely on wafer-level trimming processes. Thus, if the '516patent were to be modified with newer IC packages, the trim circuitrydescribed therein would necessarily require additional package pins thatwould be only used for trimming.

Additionally, conventional trimming process using fuses and/or zenerdiode require significant input current to effectuate blowing the fuseor zapping the diode. It is recognized that the requirement of highcurrent in an IC requires additional measures to ensure that othercomponents remain isolated from high current conditions. Additionally,such high current places large power requirements on the IC, which isundesireable.

Therefore, there exists a need to provide an on-chip, after-package trimcircuit which does not require additional external package pins canutilize package pins of the IC and which relinquishes the package pinsafter trimming, for use as proscribed by the IC. A need also exists toprovide a trim circuit adapted to be isolated from the remainder of theIC, so that the functionality of the IC is not compromised and so thatadditional components need not be incorporated into the IC to effectuatethe trim process. There also exists a need to provide a trim circuit andmethodology that is dynamically designed, and insensitive to varyingchip-to-chip tolerances of the components incorporated into the IC.

SUMMARY OF THE INVENTION

Accordingly, it is one object of the present invention to provide anafter package integrated circuit trim circuit and method which utilizesfuses for setting a trim voltage.

It is another object of the present invention to provide an afterpackage integrated circuit trim circuit and method which does notrequire additional pins to accomplish a trimming procedure.

The present invention accomplishes these and other objects by providinga programmable after-package, on-chip reference voltage trim circuit foran IC. The programmable trim circuit includes a register beingcontrolled to generate a sequence of test bit signal and a sequence ofset bit signals. A plurality of programmable trim cell circuits areselectively coupled to the register, and each cell receives a test bitsignal and set bit signal from the register. The trim cells are adaptedto generate output signals equal to said test bit signal or said set bitsignal, respectively, supplied to each trim cell. A digital to analogconverter (DAC) circuit is coupled to the output signals and generates atrim current signal proportional to the output signals. The trim currentis injected into a resistor placed between trim current generator andvoltage reference block. A trim voltage signal, with sign and magnitudeis now available for adjusting the initial, fixed, voltage referencevalue. It is added to an initial value of a reference voltage generatedby said IC.

In the preferred embodiment, the trim cell circuits include an OR gatehaving a first input and a second input, the second input beingselectively coupled to said test bit signal. The trim circuits alsoinclude a set circuit portion including a first switch having a controlnode selectively coupled to the set bit signal and a conduction node, asecond switch having a control node coupled to said conduction node ofthe first switch, an active node coupled to a common voltage railsupplied by said IC and a conduction node coupled to the first input ofthe OR gate. A nominal current source is provided in parallel with saidsecond switch and connected to the first input of the OR gate and thevoltage rail. A fuse is coupled between a ground potential rail suppliedby the IC and the first input of the OR gate. The set bit signalcontrols the conduction state of the first and second switches, and theinput value of the first input of the OR gate.

In operation, the preferred trim circuit operates as follows: if the setbit is high, both the first and second switches conduct and a conductionpath between the voltage rail and ground is established through the fusecausing said fuse to break, and thus the voltage rail is permanentlycoupled to the first input of said OR gate; and if the set bit is low,both the first and second switches do not conduct and the fuse remainsintact, and a conduction path is established between the voltage railand ground through the fuse and the current source thereby generating alow input to the first input of the OR gate.

In method form, the present invention provides method of trimming areference voltage for an integrated circuit comprising the steps of:measuring an initial value of a reference voltage signal generated bythe IC; comparing the initial value signal to a high precision referencesignal; generating a control signal indicative of the sign of differencebetween the initial value signal and the high precision signal;controlling a plurality of trim cell circuits to generate a bit sequenceindicative of a sign and magnitude of a trim current; generating a trimcurrent having a sign and value proportional to the bit sequence;generating a trim voltage by applying the trim current into a resistorplaced between trim current generator and voltage reference block;adding the trim voltage to the initial value signal and determining ifthe added signals approximately equal the high precision referencesignal; and permanently setting the bit values generated by the trimcells when the added signals equal the high precision reference signal.The preferred method also includes the step of electrically isolatingthe trim cells from the IC after the step of permanently setting saidbit values.

It will be appreciated by those skilled in the art that although thefollowing Detailed Description will proceed with reference being made topreferred embodiments and methods of use, the present invention is notintended to be limited to these preferred embodiments and methods ofuse. Rather, the present invention is of broad scope and is intended tobe limited as only set forth in the accompanying claims.

Other features and advantages of the present invention will becomeapparent as the following Detailed Description proceeds, and uponreference to the Drawings, wherein like numerals depict like parts, andwherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a detailed circuit diagram of an exemplary reference voltagetrimming circuit of the present invention;

FIG. 2 is a detailed circuit diagram depicting an exemplary testprocedure of the circuit of FIG. 1;

FIG. 3 is a detailed circuit diagram depicting an exemplary fuse burningprocedure to set the reference voltage of the circuit of FIG. 1;

FIG. 4 is a detailed circuit diagram depicting an exemplary fuse burningprocedure to isolate the trimming of the circuit of FIG. 1; and

FIGS. 5A, 5B and 5C are flowcharts of the preferred reference voltagetrimming process of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1-4 depict detailed circuit diagrams of an exemplary referencevoltage trimming circuit of the present invention. Before discussing indetail the trimming circuitry and methodology of the present invention,it should be noted that the IC 10 includes common components that areutilized herein. As will be understood by those skilled in the art,mixed signal IC packages generally include package pins that aredirected to the overall functionality of the IC. The particularfunctionality of the IC is not important to the present invention, butmay include D/A and A/D converter circuits, Op Amps, VCO circuits, etc.In the examples shown in FIGS. 1-4, the package pins utilized hereininclude VDD 28 (e.g., common voltage rail), GND 26 (e.g., ground) and aninternal reference voltage VREF that is generated by the IC that has avalue and accuracy for a particular application (i.e., functionality) ofthe package. Mixed-signal IC packages also typically include I/O signallines and package pins to operable with an internal bus controller(which, in the present example, is an SMBUS IC controller 42) forcontrolling I/O signal lines (i.e., data bus) and/or common bus pinoutsbus pins, SMBCLK 22 and SMBDATA 24. The bus (SMBCLK 22 and SMBDATA 24)depicted in FIGS. 1-4 is shown as an I²C bus configuration, but mayalternatively be a PCI bus configuration, USB, 1394, etc., or other busconfigurations known in the art. The particulars of the bus are notimportant to the present invention, provided that the bus can be used tocontrol and address an internal register 12 associated with the IC 10.It should be noted at the outset that the example shown in FIGS. 1-4 isdirected to trimming the reference voltage, as found on the VREF pin 56.The present invention is intended to provide after-package referencevoltage (VREF) trimming circuitry and methodology which does not requireadditional pins, and further, which utilizes the internal register 12only for the trimming procedure, and upon completion, releases theregister for use by the IC. Additionally, the trim circuitry includesisolation circuitry to release the package pins utilized during testing,so that the package pins can be used by the IC as intended. Thesecomponents set out above will be referred to herein, and should berecognized as being common to IC packages, regardless of thefunctionality of the IC.

Preferably, the reference voltage trim circuitry of the presentinvention is programmable to provide a best-fit trim value, and includesfuses to permanently set the best-fit value. As an overview, the presentinvention comprises three main operations: 1) determination of aninitial value of an IC reference voltage (V0) and comparing that valuewith a known, high-precision reference signal to determine the offset ofreference voltage, 2) programming trim circuitry with a bit count togenerate a trim current, to convert it into a trim voltage and increment(add or subtract) the reference voltage (V0) with the trim voltage toobtain a reference voltage to within a predetermined tolerance, and 3)permanently fixing the value of the trim signal (and thus, the referencevoltage), and isolating the trim circuitry from the internal IC registerand the package pins. Each of these operations, and the preferredcircuitry to accomplish same, are discussed separately below.

Test Interface Circuitry and Measurement of V0

For clarity the initial reading of VREF shall be referred to herein asV0. To measure the IC reference voltage V0, and for controlling the IC,via I/O lines 22, 24, to implement a test procedure, a test interface 50is provided. The test interface includes a high precision comparator 52that compares the IC reference voltage VREF (V0) to an internallygenerated, high precision reference signal Vref 72, generated bygenerator 54. To accurately define the amount of trimming necessary forVREF, it is preferable that the signal generator 54 generates areference signal 72 having an accuracy greater than the desiredresolution of VREF. The comparison of the high precision referencesignal Vref 72 and the reference signal V0 56 (at comparator 52)generates a control signal 58, indicative of the sign of V0 with respectto Vref. The test interface circuitry preferably also includes a buscontroller 62, that is appropriately adapted to write data to the buscontroller 42 (via I/O lines 22, 24). The bus test controller 62 isprovided to interface with bus controller 42 (internal to the IC) forcontrolling read/write operations to register 12, as will be describedbelow. The test interface also preferably includes a voltmeter 60 tomonitor the incremental adjustments made to V0 during the trimmingprocedure (as will be described below). Once the control signal 58 isinitially determined, it is used to control the test controller 62. Byconvention, it will be assumed herein that if V0>Vref, the controlsignal 58 will be negative (−), indicating that the trimming must reducethe value of V0. Likewise, if V0<Vref, the control signal 58 will bepositive (+), indicating that trimming must increase VREF (V0). Once VOis determined, test controller 62 (belonging to test interface 50) willcommand controller 42 (belonging to IC 10) based on the value of controlsignal 58, to begin the trim test procedure, described below.

FIG. 5a depicts the preferred flowchart 100 for the initial reading ofV0, and the determination of the sign of control signal 58. For clarity,those components set forth in FIGS. 1-4 (i.e., test interface 50including comparator 52 and high precision reference signal generator54) will be referred to below without reference numerals. Initially, thetest interface takes a reading if the initial value of the IC referencevoltage V0 102. The test interface compares V0 to a high precisionreference voltage Vref 104. To generate the control signal 110, acomparator determines if Vref>V0 112, or if Vref<VO 114. If Vref>VO, thetest interface concludes that a trim voltage must be added to V0, andthus the sign of the sign control signal is positive (+) 116.Conversely, if Vref<V0, the test interface concludes that a trim voltagemust be subtracted from V0, and thus the sign of the sign control signalis negative (−) 118. If 116, the test interface controls an internal ICregister to assign a sign bit=1 120, indicating that the sign of thetrim voltage is to be positive. If 118, he test interface controls aninternal IC register to assign a sign bit=0 122, indicating that thesign of the trim voltage is to be negative. In either case, thecontroller begins the test procedure, as to trim the reference voltageV0 to within a predetermined threshold, the preferred process of whichis depicted in FIG. 5B (described below).

Bit Count Trimming and Preferred Trim Circuitry

Turning now to the on-chip trim circuitry, and as an overview, thepreferred arrangement includes a sign designation trim cell 14A, and aplurality of trim cells 14B, 14C, 14D and 14E that are selectivelycoupled to an IC register 12. Each trim cell defines one bit, from LSBto MSB, of a bit count. Test cell 14A is designated as the sign bitgenerating cell, whose value remains constant during the bit countprocedure herein described. An additional cell 16 is provided to controlaccess and isolation of the test cells 14A-14E. For manufacturing easeand overall repeatability, it is preferred that the cells 14A-14E and 16be substantially identical (except as more fully described below). Eachtrim cell 14A-14E has two inputs from the register 12: a test input (T)and a set input (S). Accordingly, each trim cell 14A-14E preferablyincludes a test circuitry and set circuitry. Control cell 16 has asingle input from the register 12 (designated as OPEN). During the bitcount, the register 12 supplies test cells 14B-14E with the current bitcount on each respective bit test line T. Preferably, during the testprocedure, the register holds the set circuitry of each trim cellinactive by supplying a bit value of 0 thereto, until the proper bestfit bit values have been determined and the best fit values have beensimulated. The preferred circuit arrangement for trim cells 14A-14E isdescribed more fully below.

Each test cell 14A-14E preferably includes an OR gate 30, a currentsource 32 and a transistor pair MN 36 and MP 34, and a fuse 48. Eachtest cell is adapted to perform a test procedure and a set procedure. Tothat end, in the preferred embodiment of FIGS. 1-4, each test cellincludes test circuitry and set circuitry. The transistors, currentsource and fuse are coupled to a set bit input (S) from the register 12and to the OR gate. During the test operation, the set bit input is heldlow, so that this circuitry remains inactive. For testing, the test bitsignal T is directly coupled to the OR gate (via transfer gates 38) fromthe register, which are generated by the controller 42. Resistors R1, R2are provided as bias resistors for the transistors MN and MP, as isunderstood in the art, and are not essential to understanding thepresent invention. Likewise, resistor R3 is provided to prevent afloating condition at the OR gate 30 when the fuse is blown. Thetransistors are used once the correct bit value for that cell has beendetermined, for blowing the fuse 48 associated with each cell, theoperation thereof is set out below with reference to FIG. 3. TransistorMN 36 preferably comprises a low-power nMOS transistor that iscontrolled by the set bit (bit S output from register 12). The drain ofMN is coupled to the IC voltage source VDD 28 (via biasing resistor R2),and to the gate of MP 34. Transistor MP preferably comprises ahigh-power pMOS transistor having its source coupled to VDD and it'sdrain coupled to the OR gate 30. Also coupled to the source of MP (andto the drain of MN), a current source IG is provided, which is likewisecoupled to the OR gate 30, as shown, and to the fuse 48. MP is providedin parallel with IG, and both are coupled to the fuse and ground GND 26.Preferably, fuse 48 comprises a metal or polysilicon resistor. Mostpreferably, the fuse 48 is blown by electrical overstress, for example,having a maximum electrical stress (i.e., rating) value of 1A/5V formetal type resistors or 0.1A/5V for polysilicon. As is understood, thevalue of the fuse 48 is not limited to the above-example, and all suchfuse types known in the art are deemed within the scope of the presentinvention. Accordingly, the current source 32 preferably generates acurrent having a value several orders of magnitude smaller than requiredto blow the fuse 48 (for example, a 1A is sufficient for mostapplications).

During the trim process, controller 62 of the test interface 50 commandscontroller 42 of the IC 10 (via, e.g., I/O lines SMBCLK and SMBDATA) totake control of internal register 12 and begin a bit counting procedureon those bits that have been designated as test bit (T) signal lines.Additionally, the test bit input into the sign designation trim cell 14Ais held constant, and is not part of the bit count sequence. The signbit T supplied to test cell 14A is determined by control signal 58,described above. The remaining test cells, 14B-14E, receive input datafrom register 12 (via test signal lines T) reflective of the bit countsequence. It is preferred that the bit count proceeds from the LSB tothe MSB. Thus, the bit count during the test procedure proceeds from0001, 0010, 0011, 0100, . . . , 1111 (i.e., LSB to MSB) for a 4-bitresolution trimming procedure. In the examples shown, at each bit count,test bits 3, 5, 7 and 9 (in register 12) will be incremented. Thoseskilled in the art will recognize that there is no particularsignificance associated with the choice of these particular bits, but isonly provided as an example. Moreover, those skilled in the art willrecognize that any number of test cells can be provided to achieve adesired resolution, and the circuit diagrams depicted in FIGS. 1-4 aremerely shown as an example.

At each counting step, the test cells pass each bit value through to ORgate 30. The output signals of each OR gate are input into a current D/Aconverter, current DAC 18. The bit values are summed and converted intotrim current, Itrim, utilizing the current DAC 18. A voltage drop Vtrim70 is added to the initial reference value V0 to produce VREF.Accordingly, VREF is defined as function of VO (the initial value ofVREF) and the addition of the trim voltage, Vtrim (i.e.,VREF=f(V0+Vtrim)). To that end, trim resistor RO is provided and coupledto Itrim, for generating Vtrim. The value of RO is not important,provided that it remains constant throughout the procedure. The currentDAC 18 provides an analog current Itrim, whose absolute value is afunction of the test bits: bit_0, bit_1, bit_2 and bit_3 (output fromtest cells 14B, 14C, 14D and 14E, respectively). The sign of Itrim isdetermined by bit_s, which is proscribed by the control signal 58,described above.

Initially, the test cells 14A-14E are opened to accept data from theregister 12, via cell 16. Cell 16 (herein referred to as theOPEN/ISOLATE cell) controls the OPEN signal line 40. Coupled betweeneach input date line T and S from register 12 and each cell 14A-14E, aplurality of transfer gates 38A-38E are provided. The transfer gates38A-38E are controlled by the OPEN signal line 40. During the test andset procedures, the OPEN signal line controls gates 38A-38E so that datafrom register 12 is permitted to flow into the test cells 14A-14E. Toaccomplish this, cell 16 has a configuration similar to test cells14A-14E, but operates on a single input OPEN, and further includes aninverter 44. During the test and set procedure, the OPEN bit is heldlow, so that the output of the inverter is HIGH, thus opening thetransfer gates 38A-38E.

In the example of FIGS. 1-4, it will be assumed that sign signal 58 isdetermined to be negative (i.e., V0>Vref), and thus, will dictate thatthe current Itrim (and hence the trimming voltage Vtrim) be negative,and that the negative value Itrim be summed with V0. By convention, forcurrent DAC to produce a negative current, the sign bit is 0. Thus,controller 62 commands controller 42 to ensure that the register has abit value of 0 for the test bit signal line T (bit 1 in the drawings)associated with cell 14A throughout the entire test procedure. Referringnow to FIG. 2, assume that the trimming test procedure has beenunderway, and that test bits 3, 5, 7 and 9 have values of 0, 0, 1 and 1,respectively. The current DAC will generate a current Itrim based andthese bit values (which generates a proportional trimming voltage Vtrimacross resistor R0). Vtrim is added to V0 and again compared to Vref, atcomparator 58. This is done for each bit count in the sequence. Notethat bit 1=0, and thus Itrim is negative, and thus Vtrim is negative. Ifthe comparison generates a control signal 58 that has changed sign(e.g., VREF is now less than Vref), then it is determined that Vtrim hasbeen established to within the threshold of 1 LSB. Controller 62commands controller 42 to stop the bit count sequence and hold thecurrent bit sequence in the register, for emulation and setting.

In the example shown, assume the correct bit sequence is determined asbeing 0, 0, 1, 1 for test cells 14 b, 14C, 14D, and 14E, respectively(See FIG. 2). For test cell 14A it is determined that Vtrim should benegative, and thus cell 14A generates a 0 value. This is only anexample, and is not to be construed as limiting the present invention tothis particular bit sequence.

FIG. 5B depicts a flowchart 200 for the preferred trim test procedurefor obtaining the value of the trim voltage Vtrim. For clarity, thosecomponents set forth in FIGS. 1-4 (i.e., test interface 50 includingcomparator 52 and high precision reference signal generator 54, and thetrim components associated with the IC 10 including the trim cells14A-14E, isolation cell 16, register 12 and transfer gates) will bereferred to below without reference numerals. Once the sign of the signcontrol signal 58 has been determined, the test procedure begins bycontrolling the IC data bus and internal register 202 to begin a bitcount procedure. A sign bit, reflective of the sign of the sign controlsignal, is assigned in the register 204. According to a predetermineddesired resolution (i.e., bit-depth), a plurality (n) of test bits areassigned in the register 206. To access the trim cells circuits, an OPENbit is assigned in the register 208, which couples the trim cells to theregister. The register is controlled to begin a count of the test bitsin order, from LSB to MSB 210. At each count, each test bit is placedinto a corresponding trim cell 212. At each count, the test bits aresummed 214, and the summed test bits are converted to generate a trimcurrent, Itrim, 216. At step 216, the absolute value of Itrim isdetermined. The sign of Itrim is determined using the sign bit 218.Itrim is input into a resistor (R0) to generate a voltage drop, Vtrim(=Itrim×R0), proportional to Itrim. Itrim×R0 (Vtrim) is added to V0 220.A test interface reads V0+Vtrim, and compares V0+Vtrim to the highprecision reference voltage Vref 222. The test interface determines ifcontrol bit of test interface, changes the value. If so, at this point,these bit values are emulated and the trim circuitry is isolated fromthe register, as set out below with reference to FIG. 5C. If not ,thetest interface determines if Itrim is maximum for the given bitresolution 228. If so, the IC is determined bad and the procedure stops230. If not, then it is known that the current bit count has notachieved a correct value for Itrim, and the process repeats with thenext count 234. If the sign of the control signal has changed, it isdetermined that Itrim is established to within the limits of thepredetermine bit resolution (n) (i.e., to within one LSB) 236. Withthese bit values, the IC is emulated to ensure that Itrim still producesthe correct trim voltage Vtrim 238. The test interface holds these bitvalues in memory 240, which are used to set the trim cells (describedbelow with reference to FIG. 5C242).

Setting the Trim Value and Isolating the Trim Circuitry

Referring now to FIG. 3, and assuming that the correct bit sequence hasbeen determined and emulated, controller 62 commands controller 42 toshift, in the register 12, the test bits to the corresponding set (S)bit lines for each trim cell. In the example shown, bits 0 and 1 areinput as set (S) and test (T) bits into test cell 14A, bits 2 and 3 areinput as set (S) and test (T) bits into test cell 14B, and so on fortest cells 14A-14E, as shown. Thus, in this example, bit 1 is shifted tobit 0, bit 3 is shifted to bit 2, bit 5 is shifted to bit 4, and so on,for test cells 14A-14E. By shifting the bits as shown, the set circuitryassociated with each test cell is activated. As an example, theoperation of the preferred set circuitry is described below. At theoutset, it is to be understood that if a particular bit in the test cellis 1, the fuse for that bit will blow, thus permanently setting theoutput of that cell to 1. Likewise, if a particular bit in the test cellis 0, the fuse for that bit will not blow, thus permanently setting theoutput of that cell to 0.

Set Bit=1

In FIG. 3, the following description centers around test cell 14D, whichhas a set bit (S) value of 1, as shown. It is assumed that controller 62commands controller 42 to hold the OPEN signal line 40 in a conductivestate, via cell 16 and the OPEN bit input from the register 12, therebypermitting the test cells to receive data input from the register 12 viatransfer gates 38. It is also assumed that the test bits have beenshifted to the set bits. As is shown, the set bit is input into the gateof switch (e.g., transistor) MN 36, which turns MN 36 ON (i.e.,conducting). The drain of MN 36 is coupled to VDD (via biasing resistorR1) and to the gate of switch MP 34. Since the drain of switch 36 isconducting VDD, and since the drain is coupled to the gate of switch MP34, switch 34 is likewise conducting. Thus, a conduction path isestablished from VDD, through MP34, through Fuse_2 (FIGS. 1 and 2), toground GND. Fuse_2 has a rating less than VDD, thus Fuse_2 is blown, asshown in FIG. 3. Current source 32 is coupled to VDD and to OR gate 30,in parallel with switch 34. A similar operation occurs at test cell 14E,whose set bit is also equal to 1, thereby blowing Fuse_3.

Set Bit=0

In FIG. 3, the following description centers around test cell 14A, whichhas a set bit (S) value of 0, as shown. Recall that test cell 14Acontrols the sign of Itrim, via the current DAC 18. It is assumed thatcontroller 62 commands controller 42 to hold the OPEN signal line 40 ina conductive state, via cell 16 and the OPEN bit input from the register12, thereby permitting the test cells 14A-14E to receive data input fromthe register 12 via transfer gates 38. It is also assumed that the testbits have been shifted to the set bits. As is shown, the set bit isinput into the gate of switch (e.g., transistor) MN 36, and, since theset bit S=0, MN 36 OFF (i.e., not conducting). The drain of MN 36 iscoupled to VDD (via biasing resistor R1) and to the gate of switch MP34. Since the drain of switch 36 is not conducting and since the drainof switch 36 is coupled to the gate of switch MP 34, switch 34 islikewise in a non-conducting state. Thus, no conduction path isestablished from VDD, through MP34, through Fuse_sign (FIGS. 1 and 2),to ground GND. Moreover, since the current source 32 is in parallel withMP, the current source is only conducting a nominal amount of current,insufficient to blow Fuse_sign. Thus, fuse_sign remains intact, andsupplies a conduction path to ground for the current source. Thus, bothinputs to the OR gate 30 remain low, and hence, the output is low. Asimilar operation occurs at test cells 14B and 14C, whose set bit isalso equal to 0, and thus, Fuse_0 and Fuse_1 remain intact.

Once the appropriate fuses have been blown, or left intact, as describedabove, the preferred embodiment implements an isolation procedure tofree the internal register 12 for other purposes, so that an additionalregister need not be dedicated only to the trimming procedure, and forpermanently isolating the test cells so that the trim value obtainedtherefrom does not change. Accordingly, and referring now to FIGS. 1, 3and 4, test controller 62 commands bus controller 42 to change the signof the OPEN bit coupled to cell 16, in register 12. Before discussingthe preferred isolation operation, a brief description of cell 16follows. Cell 16 preferably includes an nMOS transistor 36′, a pMOStransistor 34′, a current source 32′, a fuse (Fuse_freeze) and aninverter 44. (It should be noted the drawings depict biasing resistorsassociated with cell 16 that are not essential to the understanding ofthe present invention, and are included to properly set the bias oftransistors 36′ and 34′, as is understood in the art). The gate ofswitch 36′ is coupled to the OPEN bit signal line from register 12. Thedrain of switch 36′ is coupled to gate of switch 34′ and to the sourceof switch 34′. The gate of switch 34′ is coupled to ground GND, throughFuse_freeze, and to the input of inverter 44. In parallel with switch34′ is current source 32′, having similar properties as current source32, described above. During the test and set operations, OPEN bit signalline is held LOW, thus, switches 34′ and 36′ are in non-conductingstates. Hence the input of inverter 44 is LOW, and it's output is HIGH,thereby placing transfer gates 38 in conducting states. Once the fusesare set (described above) it is preferable to isolate the test cells14A-14E, as follows. Controller 62 commands controller 42 to change theOPEN bit in register 12 from LOW to HIGH. At this time, switches 34′ and36′ are conducting, and Fuse_freeze 46 is blown, thereby isolating thetransistors 34′ and 36′. Since the current source 32′ is in parallelwith switch 34′, it controls the input to the inverter 44.

In FIG. 4, the result of this operation is depicted. The input toinverter 44 is current source 32′. The inverter preferably comprisessufficient input impedance to effectively raise the input to HIGH, viacurrent source 32′, and thus, the output signal 40 is low. Thus, all thetransfer gates 38 are OFF, thereby isolating the register 12 from thetest cells 14A-14E. Since the test cells are isolated, the valuesgenerated by each OR gate remain fixed, as set by the test and setprocedures described above. Note that, if a set bit is 0 for aparticular cell (in this example, cells 14A, 14B and 14C) the currentsource 32 is couple to ground via the unblown fuse (e.g., Fuse_sign,Fuse_0, and Fuse_1). Thus, VDD (coupled to the current source) has noeffect on the circuit and will not generate a HIGH value input into theOR gate. Those skilled in the art will recognize that the inputimpedance of the OR gate is orders of magnitude higher than theresistance of the fuse. Thus, the current from the current source flowsto ground through the fuse and does not register as a HIGH input intothe OR gate. Conversely, if a fuse has been blown (in this example, asshown in test cells 14D and 14E). The current source is coupled only tothe input of the OR gate. Likewise, VDD is coupled to the OR gate viathe current source. Thus, the input into the OR gate remains permanentlyHIGH. Thus, the trim current, Itrim, generated by the current DAC ispermanently set by these inputs from the OR gates of each test cell.Thus, as shown in FIG. 4, register 12 is completely isolated from thetest cells 14A-14E and the OPEN cell 16, and may thus be used by the ICfor other purposes. Moreover, package pins VDD 28, VREF 20, I/O pins 22,24 and GND 26 are freed for use by users of the IC.

FIG. 5c depicts a flowchart 300 for the preferred method of setting thetrim value (Vtrim) and isolating the trim circuitry from the register.For clarity, those components set forth in FIGS. 1-4 (i.e., testinterface 50 including comparator 52 and high precision reference signalgenerator 54, and the trim components associated with the IC 10including the trim cells 14A-14E, isolation cell 16, register 12 andtransfer gates) will be referred to below without reference numerals. Asset out above with reference to FIGS. 1-3 and 5B, Vref (V0+Vtrim) havebeen established to within the resolution limits 302. The test interfaceagain controls the data bus and register 304 to begin the isolationprocedure and trim voltage setting procedure. In the register, each ofthe test bits are transferred to the set bits, respectively, and thesign bit is likewise transferred to a set bit, each set bitcorresponding to one trim cell 306. In each trim cell, circuitry isprovided to burn a fuse (associated with that trim cell) if the setbit=1, including the transferred set bit of the sign bit 308. The testinterface controls the register to change the sign of the OPEN bit 310,thereby changing the state of the cell controlling access to the trimcircuitry. By changing the sign of the OPEN bit, a fuse is blown in thecell controlling the trim circuitry, thereby isolating the trimcircuitry (i.e., trim cells) from the register 312. The test interfaceagain compares VREF to Vref 314. If VREF=Vref 316 within 1LSB accuracy,the IC is deemed operable, and VREF is accurate 318. If VREF does notequal Vref within 1LSB accuracy, the IC is deemed bad, in which case anaccurate VREF signal cannot be obtained, and the chip is discarded 320.

In operation, the test bit (T) is directly coupled to the OR gate (viatransfer gate 38, described below). Likewise, the above describedarrangement of the transistors ensures that if the set bit is 0, the ORgate 30 preferably has two inputs: a test input and a set input. In theembodiment shown in the figures, the test input, T (generated byregister 12) passes through the test cell to the input of the OR gate.

Thus, it is evident that there has been provided an after package trimcircuit and method that satisfies the aims and objectives stated herein.Those skilled in the art will recognize numerous modifications that canbe made to the present invention. For example, although the detaileddescription sets fourth an exemplary bit-depth of 4 bits, it is to beunderstood that any bit depth can be chosen, depending on the desiredresolution. To that end, the additional trim cells 14A-14E would need tobe provided if a greater resolution is desired. Of course, those skilledin the art will recognize that the resolution limits may depend on thesize of the IC register 12 (although it is equally contemplated thatmore than one register can be utilized for the present invention).

Other modifications are possible. For example, the transistors MN 36 andMP 34 are described herein generically as general MOS transistors, butcould equivalently be replaced with BJT transistors, and/or otherswitches known in the art. Likewise, the current DAC 18 is preferably ageneric current DAC having inputs as shown, which generates aproportional current Itrim. It should be noted that it is assumed hereinthat the DAC generates a current Itrim that is linearly proportional tothe bit value inputs. Alternatively, the DAC can be configured togenerated a nonlinear (e.g., geometric, exponential, logarithmic, etc.)and/or weighted current value based on the inputs. The current source 32is depicted as a dependant current source, dependant upon the voltagerail VDD. Since the current source is directly couple to the fuse, it isassumed that the nominal output is insufficient to blow the fuse, butrather provides a conduction path to ground. It is further assumed thatonly a direct coupling between the voltage rail and the fuse (via switch34) is sufficient to blow the fuse. However, those skilled in the artwill recognize that this configuration can be modified using additionalcircuitry, yet generating the same result. All such modifications aredeemed covered herein

Still other modifications are possible. For example, the test interface50 disclosed herein can be provided as a separate board, having anappropriately adapted temporary IC chip holder with the pinouts for VDD,GND, VREF and the I/O bus lines, as shown. Alternatively, the testinterface can include a separate portable hand-held device having thedisclosed functionality, and having attachment means (e.g., cables,clips, etc.) to interface with the IC 10.

Other modifications will become apparent to those skilled in the art.For example, it is assumed to a large extend that the componentsprovided herein are of a generic nature, but that any of thesecomponents can be replaced with other conventional or proprietarycircuit components known in the art. All such modifications are deemedwithin the spirit and scope of the present invention, as defined by theappended claims.

What is claimed is:
 1. A programmable after-package, on-chip referencevoltage trim circuit for an IC, said programmable trim circuitcomprising: a register being controlled to generate a sequence of testbit signal and a sequence of set bit signals; a plurality of afterpackage programmable trim cell circuits selectively coupled to saidregister, each said cell receiving a test bit signal and set bit signalfrom said register, said after package trim cells being adapted togenerate an output signal equal to said test bit signal or said set bitsignal, respectively, supplied to each after package trim cell; and adigital to analog converter (DAC) circuit coupled to said output signalsand generating a trim current signal proportional to said outputsignals; a resistor to convert the trim current into a trim voltagesignal, said trim voltage being added to an initial value of a referencevoltage generated by said IC.
 2. A trim circuit as claimed in claim 1,said trim circuit further comprising an isolation trim cell operablewith a plurality of transfer switches disposed between said register andsaid trim cell circuits and receiving said test bit signals and set bitsignals from said register and supplying said test bit signals and setbit signals to said trim cells, said transfer switches being controlledby said isolation trim cell to couple and uncouple said register fromsaid trim cell circuits upon receiving an open bit or a close bit fromsaid register, respectively.
 3. A trim circuit as claimed in claim 1,wherein each trim cell circuit comprises: an OR gate having a firstinput and a second input, said second input being selectively coupled tosaid test bit signal; and a set circuit portion including a first switchhaving a control node selectively coupled to said set bit signal and aconduction node, a second switch having a control node coupled to saidconduction node of said first switch, an active node coupled to a commonvoltage rail supplied by said IC and a conduction node coupled to saidfirst input of said OR gate, and a nominal current source in parallelwith said second switch and connected to said first input of said ORgate and said voltage rail, and a fuse coupled between a groundpotential rail supplied by said IC and said first input of said OR gate;wherein said set bit signal controlling the conduction state of saidfirst and second switches, and the input value of said first input ofsaid OR gate.
 4. A trim circuit as claimed in claim 3, wherein if saidset bit is high, both said first and second switches conduct and aconduction path between said voltage rail and ground is establishedthrough said fuse causing said fuse to break, and said voltage railbeing permanently coupled to said first input of said OR gate; andwherein if said set bit is low, both said first and second switches donot conduct and said fuse remains intact, and a conduction path isestablished between said voltage rail and ground through said fuse andsaid current source thereby generating a low input to said first inputof said OR gate.
 5. A trim circuit as claimed in claim 2, wherein saidisolation trim cell comprising: an inverter circuit having an input, andan output coupled to a control node of said isolation switches andsupplying a signal thereto to determine a conduction state of saidisolation switches; and an isolation circuit portion including a firstswitch having a control node selectively coupled to an isolation bitsignal generated by said register and a conduction node, a second switchhaving a control node coupled to said conduction node of said firstswitch, an active node coupled to a common voltage rail supplied by saidIC and a conduction node coupled to said input of said inverter circuit,and a nominal current source in parallel with said second switch andconnected to said input of said inverter circuit and said voltage rail,and a fuse coupled between a ground potential rail supplied by said ICand said input of said inverter circuit; wherein said isolation bitsignal controlling the conduction state of said first and secondswitches, and the input value of said input of said inverter circuit. 6.A trim circuit as claimed in claim 5, wherein if said isolation bitsignal low, the output of said inverter is high; and if said isolationbit signal is high, both said first and second switches conduct and aconduction path between said voltage rail and ground is establishedthrough said fuse causing said fuse to break, and said voltage railbeing permanently coupled to said input of said inverter circuit therebypermanently generating a low value output from said inverter and therebycausing said isolation switches to be in a non-conducting state, therebydecoupling said register from said trim cells.
 7. A trim circuit asclaimed in claim 1, further comprising a bus controller controlling saidregister to generate said set bit signals and said test bit signals. 8.A trim circuit as claimed in claim 3, wherein one of said trim cellcircuits including a sign cell circuit and being supplied a set and testbit indicative of said control signal and generating a bit value to saidDAC indicative of a desired sign of said trim current.
 9. A method oftrimming a reference voltage for an integrated circuit, said methodcomprising the steps of: measuring an initial value of a referencevoltage signal generated by said IC; comparing said initial value signalto a high precision reference signal; generating a control signalindicative of the sign of difference between said initial value signaland said high precision signal; controlling a plurality of after packagetrim cell circuits to generate a bit sequence indicative of a sign andmagnitude of a trim current; generating a trim current having a sign andvalue proportional to said bit sequence; generating a trim voltage on aresistor placed between trim current generator and IC voltage reference;adding said trim voltage to said voltage reference value and determiningif said added signals approximately equal said high precision referencesignal; and permanently setting said bit values generated by said afterpackage trim cells when said added signals equal said high precisionreference signal.
 10. A method as claimed in claim 9, further comprisingthe step of isolating said trim cells from said IC after said step ofpermanently setting said bit values.
 11. A method as claimed in claim10, further comprising the step of controlling a register to generatesaid sequence of bit values and a sign bit based on said control signal.12. A method as claimed in claim 10, further comprising the step ofconverting said bit sequence value to a proportional analog trimcurrent.
 13. A method as claimed in claim 10, further comprising thestep of converting said trim current into a trim voltage with sign andvalue proportional with trim current value.
 14. A trim circuit asclaimed in claim 3, wherein said nominal current source being adependant current source dependant upon said voltage rail value, andgenerating a current value smaller than a rating of said fuse.
 15. Atrim circuit as claimed in claim 3, wherein said fuse having a ratingsufficient to be broken by said common voltage rail.
 16. A trim circuitas claimed in claim 3, wherein said OR gate having an input impedancegreater than the impedance of said fuse.